1. Field of the Invention
This invention relates to a water-based polyurethane primer for bonding plastics, more especially PVC.
2. Discussion of Related Art
It is known from EP 307 546 that solventless aqueous polyurethane dispersions can be used as a primer coating for plastic films based on rigid or semirigid PVC for the lamination of porous materials In addition, dispersion primers based on ethylene/vinyl acetate copolymers, ethylene/vinyl acetate rubber or acrylates are also described. A dispersion adhesive is mentioned as the adhesive. Chipboards and rigid PVC films are bonded to one another with this primer/adhesive combination.
In addition, it is generally known that PVC adherends can readily be bonded to one another with a diffusion adhesive based on tetrahydrofuran and 10 to 20% of PVC or on a two-pack adhesive such as, for example, epoxy, polyurethane, polymethyl methacrylate or unsaturated polyester (see G. Habenicht "Kleben: Grundlagen, Technologie, Anwendungen (Bonding: Basic Principles, Technology, Applications)" (1990), page 447). Unfortunately, these adhesives are attended by the disadvantage that they emit solvent and thus endanger the environment or require a longer time for setting than the machine cycle times allow. It is also generally known that, for example, PVC adherends or other adherends can be bonded to one another with a polyurethane hotmelt adhesive (see Michaeli/Netze/Freitag "Marktspiegel Kunststoffkleben" Verlag TUV Rheinland (1991), pages 323 and 324). It is also known that PVC profiles and PVC films can be bonded with a polyurethane hotmelt adhesive. The profile is prepared by treatment with chlorinated hydrocarbons, for example with methylene chloride. Accordingly, this bonding process also involves emissions and endangers the environment, all the more so insofar as PVC profiles are very widely used as solid, hollow or core profiles by virtue of their ease of production by extrusion, their low costs and their good performance properties. The PVC to be used for this purpose may be both flexible PVC and semirigid or rigid PVC. To improve the profiles and, above all, to enhance their appearance, they are surface-treated. This is generally done by lamination with a film. The films preferably consist of PVC and are generally between 0.1 and 1.0 mm thick. As with many bonded joints in the industrial sector, the lamination of PVC profiles is also determined by the production process, In other words, high initial strength in a short time and even greater ultimate strength are generally necessary.
Against the background of this prior art, the problem addressed by the present invention was to provide an adhesive system which would be solvent-free and which would bond plastics, more especially PVC profiles, to PVC films and other plastic and paper films in a reasonably quick time so that processing would not be slowed down in any way. The performance properties would of course not suffer either.